Nuclear magnetic resonance (NMR) is a useful tool in investigating the properties of a sample. More specifically, NMR tools are used in laboratories as well as in boreholes traversing earth formations in order to investigate the properties of fluids and rock samples. The NMR tools used downhole are able to generate fields that result in signals indicating the presence of water and hydrocarbons in the formation.
One frequently used NMR technique is the Carr-Purcell-Meiboom-Gill (CPMG) method of measuring transverse (spin-spin) relaxation time (T2). CPMG involves applying a series of radio-frequency (RF) pulses that tip the nuclear spins in a directional orthogonal to the equilibrium state and then uses a refocusing pulse to create successive echoes (with echo spacing Te) that reveal the magnetization decay as a function of time M(t). In the simplest case of a sample containing only one molecular species, this decay of magnetization can be analyzed as a single component having an exponential decay in time t; M(t)=M0e−t/T2. In other cases where a sample contains multiple chemical components, the decay will be multi-exponential, having multiple (n), different T2 components; M(t)=ΣinM0,ie−t/T2,i.
Other frequently utilized NMR techniques involve the determination of the longitudinal (spin-lattice) relaxation time T1 of a sample, and the determination of NMR diffusion D. Tools for conducting NMR experiments are well-known. An NMR borehole tool for making NMR measurements of NMR parameters such as relaxation times and diffusion coefficients of fluids is described in Freedman, R. et al., “A compact high-performance low-field NMR apparatus for measurements on fluids at very high pressures and temperatures”, Rev. Sci. Instrum. 85, 025102 (2014) which is hereby incorporated by reference in its entirety herein.